Hunt considered the question for a few seconds.

"You mean that in today's animals both processes are there, but all scrambled up together, but in the Oligocene species they're separated out."

"Yes."

"All the Oligocene species?" Hunt asked after a moment's further reflection. Danchekker nodded in satisfaction at seeing that Hunt was on the right track.

"Precisely, Vic. All of them."

"That doesn't really make sense. I mean, the first thing you'd think would be that some kind of mutation had occurred to change one form into the other--the scrambled-up form and the separated-out form. That could have happened either way around. In one case the scrambled form could be the'natural' terrestrial pattern that became mutated on Minerva; that would explain why the animals from there have it and the descendants of the ones that were left here don't. Alternatively, you could suppose that twenty-five million years ago the separated-out form was standard, which explains of course why the animals from that time exhibit it, but that in subsequent evolution here on Earth it changed itself into the scrambled form." He looked across at Danchekker and threw his arms out wide. "But there's one basic flaw in both those arguments--it happened in lots of different species, all at the same time."

"Quite." Danchekker nodded. "And, by all the principles of selection and evolution that we accept, that would appear to rule out the possibility of any kind of mutation--natural mutation, anyway. It would be inconceivable for the same chance event to occur spontaneously and simultaneously in many distinct and unrelated lines. . . utterly inconceivable."

"Natural mutation?" Hunt looked puzzled. "What are you saying then?"

"It's perfectly simple. We've just agreed that the difference couldn't be due to ordinary natural mutation, but nevertheless it's there. The only other explanation possible then is that it was not natural."

Impossible thoughts flashed through Hunt's mind. Danchekker read the expression on his face and voiced them for him.

"In other words they didn't just happen; they were made to happen. The genetic codings were deliberately rearranged. We are talking about an artificial mutation."

For a moment Hunt was stunned. The word deliberate denoted conscious volition, which in turn implied an intelligence.

Danchekker nodded again to confirm his thoughts. "If I may rephrase your question of a minute ago, what we are really asking is, did the animals that were shipped to Minerva change, or did the animals that were left on Earth change after the others were shipped? Now add to the equation the further fact that we have established--that somebody deliberately caused the change to happen--and we are left with only one choice."

Hunt completed the argument for him. "There hasn't been anybody around on Earth during the last twenty-five million years that could have done it, so it must have been done on Minerva. That can only mean. . ." His voice trailed off as the full implication became clear.

"The Ganymeans!" Danchekker said. He allowed some time for this to sink in and then continued. "The Ganymeans altered the genetic coding of the terrestrial animals that they took back to their own planet. I am fairly certain that the samples that were recovered from the ship at Pithead were descendants of a strain that had been mutated in this way and had faithfully carried on the mutation in themselves. This is the only logical conclusion that can be drawn from the evidence we have reviewed. Also, it is strongly supported by another interesting piece of evidence."

By now Hunt was ready for anything.

"Oh?" he replied. "What?"

"That strange enzyme that turned up in all of the Oligocene species," Danchekker said. "We know now what it did." The look on Hunt's face asked all the questions for him. Danchekker continued: "That enzyme was constructed for one specific task. It cleaved the DNA chain at precisely the point where those two coding groups were joined--in species where they were separated out, of course. In other words, it isolated the genetic code that defined the C02 -tolerance characteristics."

"Okay," Hunt said slowly, but still not following the argument fully. "I'll take your word for that. . . . But how does that support what you just said about the Ganymeans? I'm not quite--"

"That enzyme was not a result of any natural process! It was something that had been manufactured and introduced artificially. That was where the radioactive decay products came from; the enzyme was manufactured artificially and included radioactive tracer elements to allow its progress through the body to be tracked and measured. We use the same technique widely in medical and physiological research ourselves."

Hunt held up a hand to stop Danchekker going any further for the time being. He sat forward in his chair and closed his eyes for a second as he mentally stepped through the reasoning that the professor had summarized.

"Yes. . . okay. . . You've pointed out all along that chemical processes can't distinguish a radioisotope from a normal one. So, how could the enzyme have selected radioisotopes to build into itself? Answer: It couldn't; somebody must have selected them and therefore the enzyme must have been manufactured artificially. Why use radioisotopes? Answer: Tracers." Hunt again looked across at the professor, who was following and nodding encouragement. "But the enzyme does a specialized job on the modified DNA chain, and you've already established that the DNA was modified artificially in the animals that were shipped to Minerva. . . . Ah, I see . . . I can see how the two tie in together. What you're saying is that the Ganymeans altered the DNA coding of the terrestrial animals, and then manufactured a specific enzyme to operate on the altered DNA."

"Exactly."

"And what was the purpose of it all?" Hunt was becoming visibly excited. "Any ideas on that?"

"Yes," Danchekker replied. "I think we have. In fact the things that we have just considered tell us all that we need to know to guess at what they were up to." He sat back and interlaced his fingers again. "With the enzyme performing in the way that I have just described, the object of the exercise becomes clear. At least I think it does. . . . If the animals that possessed the already altered DNA were implanted with the enzymes, the chromosomes in their reproductive cells would have been modified. This would have made it possible for a strain of offspring to be bred from them who possessed the CO2 coding in the form of an isolated, compact unit that could be manipulated and'got at' with comparative ease. If you like, it enabled this particular characteristic to be separated out, perhaps with a view to its becoming the focal point of further experiments with later generations. . . ." Danchekker's voice took on a curious note as he uttered the last few words, as if he were hinting that the main implication of his dissertation was about to emerge.

"I can see what you're saying," Hunt told him. "But not quite why. What were they up to then?"

"That was how they sought to solve their environmental problem after all else had failed," Danchekker said. "It must have been something that was thought of during the later period of Ganymean history on Minerva--sometime after the Shapieron went to Iscaris, otherwise Shilohin and the others would have known about it."

"What was how they sought to solve it? Sorry, Chris, I'm afraid I'm not with you all the way yet."

"Let us recapitulate for a moment on their situation," Danchekker suggested. "They knew that the CO2 level on Minerva had begun to rise, and that one day it would reach a point that they would be unable to withstand; the other Minervan native species would be unaffected, but the Ganymeans would be vulnerable as a consequence of their breeding their original tolerance out of themselves as part of the trade-off for better accident-resistance. They lost it when they took the decision to dispense permanently with their secondary circulation systems. They declined climatic engineering as a solution and tried migration to Earth and the Iscaris experiment but both failed. Later on, it appears, they must have tried something else."